The present invention relates to a damping force control type hydraulic shock absorber provided in a suspension system of a vehicle, such as an automobile.
Hydraulic shock absorbers provided in suspension systems of automobiles or other vehicles include damping force control type hydraulic shock absorbers. These shock absorbers can control the damping force level in accordance with road surface conditions, vehicle running conditions, etc. This improves ride quality and steering stability.
In general, a hydraulic shock absorber includes a cylinder in which a hydraulic fluid is sealed. A piston, which has a piston rod connected thereto to constitute a piston assembly, is slidably fitted into the cylinder to divide the inside of the cylinder into two chambers. The piston assembly is provided with a main hydraulic fluid passage and a bypass passage, which provide communication between the two chambers in the cylinder. The main hydraulic fluid passage is provided with a damping mechanism which includes an orifice and a disk valve. The bypass passage is provided with a damping force control valve for controlling opening size of the bypass passage. A reservoir is connected, through a base valve, to one of the chambers in the cylinder to compensate for a volumetric change in the cylinder. The volumetric changes in the cylinder are caused by the extension and retraction of the piston rod by the compression and expansion of a gas sealed in the reservoir.
With the above arrangement, when the bypass passage is opened by the damping force control valve, resistance to the hydraulic fluid flowing between the two chambers in the cylinder is reduced. This reduces a damping force. When the bypass passage is closed, resistance to the hydraulic fluid flowing between the two chambers is increased, and thereby increasing the damping force. Thus, the damping force can be appropriately controlled by opening and closing the damping force control valve.
However, the above-described arrangement creates a problem. Although the damping force can be changed considerably at low piston speeds because the force is dependent on the orifice area of the bypass passage, the damping force cannot be greatly changed at intermediate and high speeds. At these speeds the damping force depends on the damping force generating mechanism in the main hydraulic fluid passage (which includes the disk valve at a predetermined valve opening pressure).
In order to solve the above-mentioned problem, a damping force control type hydraulic shock absorber in which a pressure chamber is formed at the back of a main valve has been proposed. The main valve serves as a damping force generating mechanism in a main hydraulic fluid passage provided in a piston assembly. The pressure chamber communicates with a cylinder chamber which is upstream from the main valve through a fixed orifice. Also, the pressure chamber communicates with a cylinder chamber which is downstream from the main valve through a variable orifice (as disclosed in Japanese Utility Model Application Public Disclosure (KOKAI) No. 62-155242).
In the above-mentioned shock absorber, the opening size of the passage between the two chambers in the cylinder can be controlled by opening and closing the variable orifice. A valve opening pressure of the main valve can be initially changed by changing the pressure in the pressure chamber. Thus, it is possible to control not only orifice characteristics (in which a damping force is approximately proportional to the piston speed squared), but also valve characteristics (in which a damping force is approximately proportional to the piston speed). Thus, it is possible to widen the control range for the damping.
However, in the shock absorber disclosed in KOKAI No. 62-155242, the pressure chamber is formed by slidably fitting the main valve into a valve guide. Hydraulic fluid leaks from the area between the valve guide and the main valve. This makes it difficult to obtain a stable damping force. In particular, leakage from this area is greatly affected by changes in viscosity of hydraulic fluid due to temperature changes. Variations in damping force due to temperature changes can be undesirably large. Further, machining of the sliding portions requires high accuracy which results in higher production costs.